I have about 1600 contacts in my phone. Connects to an Exchange\BES server. 2 of those contacts have identical mobile phone numbers except for one digit in the first 4 for eg

Contact 1 - 0409 559966
Contact 2 - 0419 559966

When I go to the recently called\received numbers (i.e. press the send button from home screen) and select Contact 1 and choose SMS Contact 1 the SMS automatically puts contact 2 in the To: field. When I select show number it shows the correct mobile number i.e. 0409559966. If I change contact 2's number to 0419 779966 it stops the above mentioned behaviour. I have tried deleting contact 2 and recreating the contact but it still happens. If I open the SMS folder and choose SMS and manually select Contact 1 the above mentioned behaviour does not happen.

If you can duplicate the behavior each time either the contact need to be deleted and re added to just clear any problems out or you have found a bug in the OS. If deleting and recreating the contacts does not solve the problem, make sure you have the latest OS from your carrier. if that still does not work or you are running the most recent OS then report the bug to RIM.